<?php
$data = array(
    array(
        'order_num' => '111',
        'chidren' => array(
            array('healthy_collector_number' => '2'
            ), array(
                'healthy_collector_number' => '3'
            )
        )
    )
);
foreach ($data as $key => $value) {
    foreach ($value['chidren'] as $key3 => $value3) {
        foreach ($value3 as $key4 => $value4) {
            if (isset($data[$key][$key4])) {
                $data[$key][$key4] = $data[$key][$key4] . ',' . $value3[$key4];
            } else {
                $data[$key][$key4] = $value3[$key4];
            }
        }
    }
    unset($data[$key]['chidren']);
}

echo "<pre>";
var_dump($data);
echo "</pre>";
die();